Ukrainian police say they have identified people who abducted the journalist Georgiy Gongadze, whose death in 2000 shocked the nation.
President Yushchenko has pledged to find Gongadze's killers
Investigators are still trying to find those who ordered the abduction, Interior Minister Yuriy Lutsenko said.
Earlier Mr Lutsenko offered immunity from prosecution for intelligence officers who had shadowed and eavesdropped on Gongadze.
Many Ukrainians believe the killing was politically motivated.
Gongadze's headless body was found buried in a forest after his abduction in Kiev in September 2000.
Ukraine's new liberal president, Viktor Yushchenko, has told Gongadze's mother it is "a matter of honour" for him to find her son's killers.
Hunt for organisers
A proper investigation into Gongadze's murder was among the key demands of Mr Yushchenko's supporters when they were in opposition to his predecessor, Leonid Kuchma.
"We have clearly established who specifically put Gongadze in his last car. What is more, this car is also in our hands," Mr Lutsenko told Ukraine's ICTV private channel.
He said he was now less interested in those who committed the crime, but more in "those who organised and ordered it".
A few hours after Mr Lutsenko's interview, a man who the Ukrainian media describe as a key witness in Gongadze's case was attacked. The man and his guard were reportedly injured after an unknown assailant threw a grenade at them.
Ukrainian sources say that two of the four witnesses in the case - all of them members of a criminal gang charged with other crimes - have already been killed, one of them while in prison.
Meanwhile, Ukrainian investigators have started studying an audiotape of a conversation which alleges that Kremlin election adviser Gleb Pavlovsky played a role in the poisoning of Mr Yushchenko, AFP reports, quoting a spokesman for the prosecutor-general.
The poisoning, which happened in the heat of the Ukrainian presidential campaign last autumn, severely disfigured Mr Yushchenko's face.
Mr Pavlovsky described the allegations in an interview with AFP as "a gross fabrication".